1. Field of the Invention
The present invention relates to a torque distributing mechanism for a differential comprising a single input element and two output elements, wherein a torque applied to the input element of the differential is distributed to the two output elements at a predetermined ratio.
2. Description of the Prior Art
The differential provided in a power transmitting system in an automobile is arranged to absorb a difference in rotational speed produced between left and right wheels during turning of the automobile and to distribute a torque of an engine to the left and right wheels at an appropriate ratio. However, the commonly used differential is operated by a difference between loads applied the left and right wheels and hence, suffers from a problem that when one of the wheels runs onto a road surface having a smaller friction coefficient and such wheel races, the amount of torque transmitted to the other wheel is decreased, or the transmission of the torque is blocked.
In order to avoid such disadvantage, a torque distributing mechanism for a differential has already been proposed by the present applicant (see Japanese Patent Application Laid-open No. 321435/92), and which is designed to positively control the differential on the basis of an angle of rotation of a steering wheel and/or a vehicle speed to distribute a torque suitable for the operational condition to the left and right wheels.
FIG. 5 illustrates the structure of a conventional torque distributing mechanism for a differential. As shown in FIG. 5, a planetary gear type differential D, to which a driving force of an engine E is transmitted through a transmission M, includes a ring gear 01 serving as an input element, a planetary carrier 02 serving as a first output element, and a sun gear 03 serving as a second output element. The planetary carrier 02 is connected to a right wheel W.sub.R, and the sun gear 03 is connected to a left wheel W.sub.L. A planetary gear mechanism P for distributing torque to the right and left wheels W.sub.R and W.sub.L includes a sun gear 05 driven by a motor 04, a planetary carrier 06 coupled to the sun gear 03 of the differential D, and a ring gear 07 connected to the planetary carrier 02 of the differential D. The connection of the planetary carrier 02 with the ring gear 07 is achieved by allowing a pair of spur gears 010 and 011, coupled coaxially, to be meshed with an external-tooth gear 08 provided on the planetary carrier 02 and an external-tooth gear 09 provided on the ring gear 07, respectively.
The torque of the engine E is transmitted to the right and left wheels W.sub.R and W.sub.L at a predetermined ratio by driving the motor 04 for normal and reverse rotations at a predetermined rotational number.
However, the above-described prior art torque distributing mechanism for the differential is accompanied not only by a problem that a diametrical dimension is increased by a value corresponding to the spur gears 010 and 011, because the ring gear 09 of the planetary gear mechanism P and the planetary carrier 02 of the differential D are connected to the torque distributing mechanism through the pair of spur gears 010 and 011, but also by a problem that the total weight is increased, because of a large torque is applied to the spur gears 010 and 011. Further, the prior art mechanism has a problem that the degree of freedom in design is restricted, because it is necessary for the radius r.sub.1 of the external-tooth gear 09, provided on the ring gear 07, and the radius r.sub.2 of the external-tooth gear 08, provided on the planetary carrier 02, to satisfy a relation EQU r.sub.2 /r.sub.1 =1+(Zs/zr) (1)
wherein Zs and Zr represent the numbers of teeth of the sun gear 05 and the ring gear 07 of the planetary gear mechanism P, respectively.
In order to reduce the size of such torque distributing mechanism for the differential, it is desirable to use a small-sized and lightweight motor. However, the small-sized and lightweight motor only delivers smaller torque and hence, it is necessary to increase the reduction ratio to amplify the torque.
In order to amplify the torque of the motor 04 in the prior art torque distributing mechanism for the differential, it is necessary to reduce the number of teeth of a pinion 012, provided on an output shaft of the motor, and to set the number of teeth of an external-tooth gear 013, provided integrally with the sun gear 05, at a large value. However, there is a limit to reduce the number of teeth of the pinion 012 and moreover, there is a problem that if the number of teeth of the external-tooth gear 013 is increased, the diametrical dimension of the torque distributing mechanism is increased.